1. Field of the Invention
The present invention relates to the modification of an inorganic material and a modified inorganic material. In particular, the present invention relates to a composite electrolyte membrane comprising a modified inorganic material, and a fuel cell comprising the composite electrolyte membrane.
2. Description of the Related Art
Direct methanol fuel cells (DMFCs) use an aqueous methanol solution as fuel. DMFCs can easily be operated at room temperature and may have a small size. As a result, DMFCs can be used as power sources for various applications including pollution-free electric cars, domestic power generating systems, mobile communications, medical equipment, military use, space-related businesses and portable electric devices.
FIG. 1 illustrates the conventional structure of a DMFC. Referring to FIG. 1, a DMFC includes an anode 20 to which fuel is provided, a cathode 30 to which an oxidant is provided, and an electrolyte membrane 10 interposed between the anode 20 and the cathode 30. In general, the anode 20 includes an anode diffusion layer 22 and an anode catalyst layer 21, and the cathode 30 includes a cathode diffusing layer 32 and a cathode catalyst layer 31. A separator 40 includes a channel through which fuel is provided to the anode 20, and acts as a conductor by transporting electrons generated in the anode 20 to an external circuit or a neighboring unit cell. A separator 50 includes a channel through which an oxidant is provided to the cathode 30, and acts as a conductor by transmitting electrons provided from an external circuit or a neighboring unit cell to the cathode 30. In DMFCs, an aqueous methanol solution is provided to an anode as a fuel, and atmospheric air is provided to a cathode as an oxidant.
The aqueous methanol solution passes through the anode diffusion layer 22 and then arrives at the anode catalyst layer 21 where it decomposes into carbon dioxide, electrons, protons, etc. The protons pass through the electrolyte membrane 10, and then arrive at the cathode catalyst layer 31. The electrons are transported to the external circuit. The carbon dioxide is discharged to the outside. The protons, the electrons, and the oxygen from atmospheric air transmitted through the cathode diffusing layer 32 react with each other in the cathode catalyst layer 31 to produce water.
In the DMFC, the electrolyte membrane 10 acts as a proton conductor, an electron insulator, and as a separator. The electrolyte membrane 10 prevents unreacted fuel from flowing to the cathode 30 and prevents unreacted oxidant from flowing to the anode 20. Thus, the electrolyte membrane 10 acts as a separator.
An electrolyte membrane of a DMFC may comprise a polymer electrolyte with a cation exchange capacity such as a highly fluorinated polymer with sulfonate groups (e.g., Nafion®). Such polymers have a main chain comprising a fluorinated alkylene and a side chain comprising a sulfonic acid group-terminated by a fluorinated vinyl ether. The polymer electrolyte membrane retains a sufficient amount of water, thus exhibiting excellent ionic conductivity.
In addition, the polymer electrolyte membrane can diffuse methanol as well as water. In DMFCs, an aqueous methanol solution is provided to an anode as a fuel. Some unreacted methanol in the aqueous methanol solution can permeate into the polymer electrolyte membrane, causing the polymer electrolyte membrane to swell. In addition, methanol permeated into the polymer electrolyte membrane also diffuses into a cathode catalyst layer. The term “methanol crossover” indicates a phenomenon whereby methanol fed to an anode passes through an electrolyte membrane and then arrives at a cathode. The methanol crossover causes direct oxidation of methanol at a cathode, where a proton and oxygen should be electrochemically reduced. Therefore, the cathode potential decreases, and the DMFC performance can be substantially deteriorated.
Research into a method to suppress the methanol crossover at the polymer electrolyte membrane has been conducted. For example, U.S. Pat. Nos. 5,919,583 and 5,849,428 disclose a composite electrolyte membrane that is fabricated by diffusing an inorganic filler into a polymer electrolyte matrix.
Although the composite electrolyte membrane exhibits low methanol permeability, it also has poor proton conductivity due to the presence of the inorganic filler. The inorganic filler has a poor cation exchange capacity which means that as the concentration of the inorganic filler in the composite electrolyte membrane increases, the permeability of methanol decreases. Thus, conventional technologies do not provide a composite electrolyte with a cation exchange capacity that is superior to that of the Nafion® membrane.